Accompanying with the development of integrated circuits, the line-width of ultra-large-scale integrated circuit is smaller than quarter micron meters. A traditional photolithography process is applied to the formation of quarter-micron-meters lines but a line being narrower than that. In the traditional deep ultraviolet (DUV) photolithography process, an isolated-line pattern has a smaller depth of focus (DOF) than a dense-line pattern. Some approaches are tried to solve the above issue for the isolated-line pattern.
A next-generation DUV stepper machine uses a light, which has a wavelength about 0.193 nanometers, to expose photoresist. At the recent years, the DUV stepper machines, which are popular used in factories, use light that has a wavelength about 0.248 nanometers. Using the next-generation stepper machine, the DOF of the isolated line will be increased. Nevertheless, the new stepper machine is not developed very well.
Other approach to increase the DOF of the integrated circuits is to use a phase-shifting mask during a photolithography process. A phase-shifting mask (PSM) is consisted of several masking layers and attenuated layers. The phase-shifting mask has a special structure in order to an electrical field, which has a good contrast, being applied on the mask when the light of a stepper machine transmits the mask. However, the cost for manufacturing a phase-shifting mask is so expensive. Thus, the phase-shifting mask is not popular with factories. Additionally, sub-resolution assisted features are placed around an isolated line on a mask so that the DOF of the isolated line could be increased. To fabricate the assisted features on a mask needs complex calculations and designs. So, this approach is not available for semiconductor's factories.
A surface pretreatment is used to harden the surface of a DUV photoresist layer. Conventionally, the surface pretreatment is performed by using DI water or developer and it is named a liquid-phase pretreatment. The photoresist layer is dipped in DI water or developer before the developing process of the photoresist layer. But, the liquid-phase pretreatment is rough to the DUV photoresist layer. The profile of the DUV photoresist of an isolated line, which is exposed, after the liquid-phase pretreatment is T-shape. As the isolated-line DUV photoresist layer is T-shape, the critical dimension of the DUV photoresist is not definitely determined. Besides, the liquid-phase pretreatment is not uniform enough. Referring to FIG. 1, a cross section's view of a photoresist layer 10 not being treated is shown. In spite of the photoresist layer 10 has a sharp profile but the top surface of the photoresist layer 10 are round and the critical dimension of the isolated line under the photoresist layer 10 will be hard to control during following etching processes. Referring to FIG. 2, a cross sectional view of a DUV photoresist layer 10, which is treated by using a liquid-phase pretreatment and is developed, is shown and it is a T-shape.
According to the above discussion, a new surface pretreatment for the DUV photoresist is needed to increase the DOF of an isolated-line pattern.